U.S. Pat. No. 2,153,553 issued Apr. 11, 1939 discloses the high pressure polymerization of ethylene. The patent sets forth four conditions that must be met for the polymerization of ethylene to solid polymers. The pressure should be above 500 atmospheres, the temperature should be between 100 and 400° C. The oxygen content of the reactants must not exceed a very small critical amount, and there must be sufficient thermal control of the reaction to avoid undue rise in temperature. The patent then contains a caution. If the four conditions are not properly met there is either no reaction or there is an undesired reaction of explosive violence giving rise to carbon and methane. The latter is sometimes euphemistically referred to as a “decomp”.
During polymerization it has been observed that there may be a period where heat transfer from the reaction to the cooling jacket is impaired. Given the warning above it is imperative to either not impair the cooling capacity or to quickly restore the cooling capacity of the cooling jacket if it is impaired. It is believed that this loss of cooling efficiency is caused by deposition of polymer on the internal reactor wall. The current methods of removing polymer deposits from the internal reactor wall are primarily physical (i.e. send a pressure shockwave along the tube to dislodge any polymer residue).
Additionally, some operators may cycle the jacket temperatures from normal set points to about 150-200° C. for a several hours, in an attempt to dissolve or shed polymer deposits on the internal walls of the reactor.
In the operation of a high pressure polyethylene plant there are a number of conflicting desiderata. To achieve highest conversion one may set a high difference in temperature between the reactants and the cooling medium in the reactor shell (jacket) temperature. On the other hand, one may want to know at what temperatures polymer is likely to precipitate on the internal wall of the reactor leading to a number of potential consequences including reactor wall fouling, loss of cooling, formation of gels and ultimately a decomposition of the contents of the reactor.